1. Field of the Invention
The present invention relates to plasma ion nitrided (ionitrided) steel plates for use in pressure applications and, more particularly, to an improved highly durable plasma nitrided stainless steel press plate having a high-quality microfinish and methods for the manufacture of same for use in the production of wear resistant decorative laminate.
2. Description of the Prior Art
The manufacture of decorative laminate has undergone a series of innovations which have lead to greater and greater consumer expectations regarding decorative laminate durability and resistance to mar, scratch, scuff and abrasive wear. Recent efforts to produce such a wear-resistant decorative laminate, especially high pressure decorative laminate, have included the use of extremely hard alumina grit of varying sizes incorporated within the laminate surface. Whereas in the past, formulations for some decorative laminates comprising 6 micron grit as 1 percent by weight in liquid resin and 15 micron grit as 0.5 percent by weight in liquid resin have been utilized, current product trends indicate that formulations having up to 30 micron grit as 9 percent by weight in liquid resin may be necessary to meet consumer laminate wear expectations.
Press plates used to produce decorative laminates are somewhat unique in overall geometry. Manufactured from various grades of steel, particularly stainless steel, the press plate is a flat sheet of rectangular cross section and often has comparatively large longitudinal and transverse dimensions, for example, as large as sixteen and five feet, respectively. The press plates, while thus having large planar surface areas, are only about one eighth of an inch thick.
In a polished condition, the press plates ideally take on the appearance of a mirror-like sheet due to an extremely uniform planar surface, or microfinish, where microscopic discontinuities are minimized. Indeed, in the case of polished press plates, press plate microfinish quality can be determined by viewing reflected images on its surface and scrutinizing the reflected images for optical discrepancies.
Textured press plates, produced by mechanically shot peening or chemically etching their planar surface, or combinations thereof, are usually of much lower gloss than polished plates, such that instrumental gloss measurement rather than visual reflectivity is usually the primary method of characterizing their quality, although certain defects are also evident with visual inspection. Instrumental gloss measurements, in ISO or NEMA gloss units, are typically established by the manufacturer of the laminate based upon consumer expectations. The laminate gloss level in turn is directly related to the gloss of the press plates from which it is produced. The greater the gloss of the plate, the more apparent plate wear becomes.
Also, as a large flat surface used to impart a surface finish to a cellulose supported viscous resin matrix, the press plate must be free from warpage to the maximum extent possible. Warpage generally takes two forms. The first is a regular bow occurring over the entire longitudinal or transverse dimension. At modest levels, this bow is tolerable so long as the press plate assumes a nearly perfect planar orientation under the pressure of the press, which is normally in the range of 1000 to 1600 psi (6.9 to 11.0 N/mm.sup.2). The second type of warp manifests itself as localized distortions and buckling, with variations in the relative height of the press plate from a hypothetically perfect planar surface. This second type of warpage is entirely objectionable as it often does not correct itself under the pressure of the press and thus often results in defective laminate appearance and scrapping of the press plate. Both types of warpage, caused by stress relief or uneven heating, often accompanies efforts to harden the press plates by conventional means.
Thus, the level of manufacturing precision required to fabricate and maintain an overall smooth microfinish and warp-free surface, on both sides of the press plate, is critical. For example, press plates are generally used in a sandwich configuration with two composites of laminate resin-impregnated papers placed therebetween, facing opposite directions. Multiple layers of interleaved laminate material and press plates, so-called "packs" or "books", are then loaded into a press for thermal curing and pressure treatment consolidation. If excess warpage of the first type or any warpage-of the second type exists in the press plate, as well as imperfections in the surface microfinish, significantly deleterious effects on the finished decorative laminate appearance will be apparent.
However, the use of alumina grit to improve the wear-resistance of decorative laminate, even in the lower grit sizes and concentrations used in the past, destroys the surface microfinish of the conventional steel press plates heretofore employed to fabricate decorative laminates.
The physical interaction of the formulation grit and the surface of the press plate causes microscratching and resulting lower gloss, haziness, "soft glow" high spot texture wear and at times metal rub-off. Further, as the surface microfinish of the press plates imparts its overall surface finish to the final laminate product, for example, to form a high gloss or textured surface finish in the decorative laminate, any marring of the surface microfinish of the press plates renders the press plates unusable and often requires the damaged press plates to be refurbished at considerable expense or ultimately scrapped.
Attempts to use press plates of greater surface hardness fail to yield a technically and economically viable solution. Conventional polished stainless steel press plates suffer unacceptably severe microscratching after just one press run with any size alumina grit-. Textured stainless steel press plates are also easily microscratched by alumina grit. Although not as visually apparent, as with highly polished plates because of their much lower initial gloss level and inherent texture structure, the resultant gradual deterioration in gloss and texture erosion, particularly with use of larger grit sizes and concentrations, necessitates frequent refinishing.
If these stainless steel plates are hardened by conventional heat treating methods, the plates become too brittle, stress fractures can occur, and warpage becomes a significant problem. Chrome plated steel press plates also suffer from severe microscratching after relatively few pressings. Chrome plating and post-baked electroless nickel deposition on stainless steel plates have been used, yet do not satisfactory resolve the problems of grit-related microscratching and plate wear. Further, buffing and polishing operations used for polished plates or shot-blasting refinishing operations used for textured plates tend to remove the thin plated layer unevenly, causing considerable cost to re-plate the surface. The trend toward greater concentrations of even greater sized grit formulations only exacerbates these problems.
Heretofore, ferrous based alloys have been surface hardened by various treatments involving the deposition and diffusion of additional elements and compounds into the base material, notably nitrogen and carbon. However, the wide variety of industrially practiced methods used to case harden stainless steel parts are suspectable to size restrictions and high processing temperatures, often requiring subsequent oil or water quenching, which can result in unacceptable surf ace finishes and part warpage. Thus, these alternatives are impractical for treating large, relatively thin press plates.
The present invention unexpectedly has found that a concept known as plasma ion nitriding overcomes deficiencies inherent in known press plate hardening means and for the first time enables the manufacture of press plates for use in the production of wear resistant laminates containing concentrations of large alumina grit. Many applications of plasma ion nitriding techniques have been applied to significantly smaller articles or larger articles with relatively small surface to volume ratios where the final microfinish has not been a critical cosmetic aspect of the article, such as via the MPT GmbH Plasma-Triding.RTM. process with an automated control and arc discharge suppression system, which regulates the plasma input energy for better control of the quality of work treatment. None of these applications suggested that plasma ion nitriding would be a solution to the problems solved by the present invention.
Plasma ion nitriding is based on plasma discharge physics and operates by exposing a negatively charged metal work piece surface to positively charged nitrogen ions. Under vacuum in a sealed vessel, an electrical potential is applied to the system, wherein the vessel becomes the positively charged anode (electron receptor) and the work piece forms the negatively charged cathode (cation receptor). High voltage energy is used to strip electrons from nitrogen bearing gas molecules introduced into the vessel, forming a plasma, where the nitrogen ions are accelerated toward the work piece. The impact of the nitrogen ions on the surface of the work piece generates heat energy from the conversion of kinetic energy to potential energy. As the nitrogen ions impact the work piece surface, iron atoms predominantly are sputtered off at the point of impact to combine with other nitrogen ions forming iron nitride ions above the work piece surface in the glow discharge "seam". These iron nitride ions then impact and deposit on the heated work piece surface and diffuse into the subsurface molecular boundaries, creating an exposed surface layer and a distinct subsurface structure offering many of the desired characteristics for press plates as noted above, such as high hardness without brittleness, an unmarred surface finish, and a determined case depth.